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During the hot rolling process, only a very small area of
the roll pass groove surface is directly in contact with the
hot bar or rod. Different points on the groove surface go through a heating and cooling cycle. See Figure 1 for the temperature profile at any point during one revolution of the roll.
If the roll is not cooled during the rolling process, or if the
cooling is not efficient, the maximum roll temperature, the average
roll body temperature
and the temperature difference (DT) can steadily increase with time. In
addition, temperature gradients can develop within the roll.
The
alternate expansion and contraction caused by the heating
and cooling cycle, coupled with
the temperature gradients, can cause large stresses to build
up in the surface layer in the pass groove. Such stresses
can lead to the
formation of a network of thermal cracks, often referred to
as “heat-check” or “fire” cracks.
Because cemented
carbides and P/M tool steels are ultrahard materials, they are par-ticularly
sensitive to thermal cracking. Therefore, efficient cooling of the
rolls is critical to obtaining optimum performance levels from such
rolls.
Cooling of cemented
carbide and tool steel rolls can be easily and conveniently per-formed
by spraying the pass groove sur-face with a stream of high pressure
water. The volume and pressure of water, as well as the direction
of the water stream(s), will depend upon a variety of process parame-ters
which includes, but is not limited to, size and shape of pass, temperature
of material being rolled, rolling speed, area reduction, mass of roll,
thermal conductivity and thermal cracking resistance of the roll material.
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Based on experience, SinterMet recommends the
following guidelines be used during the design of cooling systems:
- The temperature of the cooling water should
not exceed the ambient temperature by more than 10°F
(6°C).
- Pass groove surface temperature should
not exceed cooling water temperature by more than 25°F
(14°C).
- Width of water stream is typically twice
the width of the pass groove (as shown in Figure 2(a)).
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Whenever possible the spray pattern should be designed to match the profile
of the pass (as shown in Figure 2(b)).
| Pass
Geometry
|
Volume
of Water per Strand per Roll (GPM) |
| Rounds/Ovals |
|
0" to
.5" wide
|
80
|
.501" to
1" wide
|
90
|
1.001" to
1.5" wide
|
100
|
1.501" to
2" wide
|
110
|
| Dogbones/Slitters |
|
Two
stranded
|
180
|
Three
stranded
|
270
|
Four
stranded
|
360
|
| Angle
Finishers/Leaders |
|
1" x
1"
|
105
|
1.5" x
1.5"
|
125
|
2" x
2"
|
140
|
| Flats |
55
GPM per 1" width
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Notes:
1) Water pressure to be 40-100 PSI at headers
2) Approximately 30% of volume of water to be delivered to bar exit.
3) Contact SinterMet for confirmation of recommended volume of water
and pressure
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Chlorides (Ions
mg/L) 40 Max
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Sulfates (Ions
mg/L) 75 Max
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Nitrates (Ions
mg/L) 3 Max
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CaCO3 (Ions
mg/L) 400 Max
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Suspended Solids(Ions
mg/L) 80 Max
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pH 8 to 8.5
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SinterMet has developed a new and improved design
for water headers (shown in Figure 3). The design incorporates the
following advanced features:
- A separate high pressure nozzle which directs approximately
30% of water volume at the bar exit.
- Side shields made of plexiglass (or other suitable material)
to help concentrate the flow of water on to the pass groove
surface.
- Slots, ports or jets cut in the header which help to direct
a stream of water at an angle on to the groove surface. This
helps the water to cascade down the groove surface and hence
to increase the contact time (i.e., heat transferred) between
the water and the roll.
The header design shown in Figure 3 (below) helps
to greatly improve cooling efficiency. Note the angular overage
from bar exit for slightly
more than 90°.
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